With rapid development of electronic industries, a diversity of printing apparatuses such as copiers, printers, scanners or multifunction peripherals become essential information apparatuses in offices. Among these printing apparatuses, the printers are the most popular. In the early stage, a printer can perform a single-side printing operation. For performing duplex printing operations by such a printer, a first page of a document file shown on a computer screen is firstly printed on a first side of a blank paper. After the first page of a document file has been printed, the paper containing the printed image of the first page should be manually turned over and then placed on the paper input tray of the printer. Next, a second page of the document file is printed on a second side of the paper. However, the process of manually turning over the paper is troublesome. This drawback becomes more serious if the document contains a great number of pages. Recently, a duplex printing apparatus has been developed for automatically performing a duplex printing operation.
FIG. 1 is a schematic cross-sectional view illustrating inner components of a conventional duplex printing apparatus. As shown in FIG. 1, the conventional duplex printing apparatus 100 principally comprises a paper input tray 101, a paper ejecting tray 102, a paper pick-up roller 106, a paper feeding channel 105, a first paper transfer roller assembly 109, a scanning and developing print unit 103, a print region 104, a paper ejecting channel 107, a heating roller 1038, a pressing roller 1039, an inverting channel 108, a paper guide slice 113, a second paper transfer roller assembly 110, a third paper transfer roller assembly 111, a fourth paper transfer roller assembly 114 and a paper ejecting roller assembly 112. The scanning and developing print unit 103 is disposed in the print region 104. The scanning and developing print unit 103 includes a laser scanning unit 1031, an optical photoconductive drum 1032, a charging roller 1033, a developer roller 1034, a toner adding roller 1035, a transferring roller 1036 and a blade 1037. The blank papers (not shown) to be printed are placed on the paper input tray 101. The printed papers are supported on the paper ejecting tray 102. The paper feeding channel 105 is arranged between the paper input tray 101 and the print region 104. The first paper transfer roller assembly 109 is disposed in the paper feeding channel 105. The paper ejecting channel 107 is arranged between the print region 104 and the paper ejecting tray 102. The heating roller 1038, the pressing roller 1039 and the paper ejecting roller assembly 112 are disposed in the paper ejecting channel 107. The paper ejecting roller assembly 112 includes a paper ejecting driving roller 1121 and a paper ejecting follower roller 1122. The inverting channel 108 is extended from the paper ejecting tray 102 to the paper feeding channel 105, as can be seen in FIG. 1. The paper which is fed into the inverting channel 108 can be transported by the second paper transfer roller assembly 110, the third paper transfer roller assembly 111 and the fourth paper transfer roller assembly 114 into the print region 104 again.
Hereinafter, a procedure of performing a duplex printing operation by the conventional duplex printing apparatus 100 will be illustrated with reference to FIG. 1. First of all, the image of the first page of the document file is read and transmitted to the laser scanning unit 1031. Then, the charging roller 1033 uniformly charges the outer surface of the optical photoconductive drum 1032. After the charging procedure, the laser scanning unit 1031 linearly scans the image of the first page in a form of laser beams, thereby forming an electrostatic latent image of the first page on the optical photoconductive drum 1032. Next, the toner adding roller 1035 supplies the developer roller 1034 with toner from a toner cartridge (not shown) of the scanning and developing print unit 103. Then, the developer roller 1034 contacts with the optical photoconductive drum 1032 for supplying the electrostatic latent image on the optical photoconductive drum 1032 with toner. As a consequence, the electrostatic latent image formed on the optical photoconductive drum 1032 is rendered visible as a toner image. After the above image processing procedure in the scanning and developing print unit 103, the paper pick-up roller 106 transports a paper from the paper input tray 101 into the paper feeding channel 105. Next, the first paper transfer roller assembly 109 transports the paper into the print region 104. When the paper passes through the region between the optical photoconductive drum 1032 and the transferring roller 1036 of the print region 104, the paper is attracted onto the surface of the optical photoconductive drum 1032 and contact with the toner. Since the outer surface of the transferring roller 1036 and the toner are oppositely charged, the toner on the optical photoconductive drum 1032 will be adsorbed onto the first side of the paper. After the toner image is transferred to first side of the paper, the blade 1037 will remove the toner remaining on the optical photoconductive drum 1032 for reuse. Next, the paper with the toner image is transported into the paper ejecting channel 107. When the paper is transported across the region between the heating roller 1038 and the pressing roller 1039, the paper is heated and pressed by the heating roller 1038 and the pressing roller 1039 such that the toner is molten and penetrated into the space between fibers of the paper. Since the heating roller 1038 and the pressing roller 1039 can facilitate fusing the toner onto the paper, the heating roller 1038 and the pressing roller 1039 function as fusing units of the typical laser printing apparatus. After the printing operation on the first page of the document file is completed, the paper containing the printed first page is transported to the paper ejecting tray 102 by the paper ejecting roller assembly 112. Meanwhile, the paper guide slice 113, which is arranged at the junction between the paper ejecting channel 107 and the inverting channel 108, is switched to close the path leading to the inverting channel 108.
Until the tail edge of the paper is nipped between the paper ejecting driving roller 1121 and the paper ejecting follower roller 1122 of the paper ejecting roller assembly 112, the paper guide slice 113 is switched to close the path leading to the paper ejecting channel 107. Meanwhile, the paper ejecting driving roller 1121 is rotated in a reverse direction and cooperates with the paper ejecting follower roller 1122 to feed the paper into the inverting channel 108. After the paper is fed into the inverting channel 108, the paper is successively transported by the second paper transfer roller assembly 110, the third paper transfer roller assembly 111 and the fourth paper transfer roller assembly 114 into the paper feeding channel 105. Next, the first paper transfer roller assembly 109 transports the paper into the print region 104. At this moment, the image of the second page of the document file has been read and transmitted to the laser scanning unit 1031. During the paper is transported across the print region 104, developing and printing operations as described for the image of the first page are performed on the second side of the paper. After the toner image is transferred to second side of the paper, the paper with the toner image is exited from the print region 104 and fed into the paper ejecting channel 107. When the paper is transported across the region between the heating roller 1038 and the pressing roller 1039, the paper is heated and pressed by the heating roller 1038 and the pressing roller 1039, which function as fusing units. After the heating and pressing operation has done, the paper is transported to the paper ejecting tray 102 by the paper ejecting roller assembly 112 and thus the duplex printing operation is completed. Meanwhile, the paper guide slice 113 is switched to open the path leading to the paper ejecting channel 107.
Although the use of the conventional duplex printing apparatus becomes more convenient because it is not required to manually turn over the paper, there are still some drawbacks. For example, during the process of turning over the paper by the conventional duplex printing apparatus, the paper should be transported across the inverting channel and sent to the print region again. The arrangement of the inverting channel increases the length of the duplex printing apparatus. Therefore, the overall volume of the duplex printing apparatus is increased, which is detrimental to the space utilization. Moreover, since the inverting channel is relatively longer, it is time-consuming to print an image on the second side of the paper and the duplex printing operation has a reduced printing speed.